This undated photo provided by the Milwaukee County Sheriff shows Sylville K. Smith. Smith, whose killing by police touched off rioting in Milwaukee, was shot by a black officer after turning toward him with a gun in his hand, the police chief said Sunday, Aug. 14, 2016, as Wisconsin's governor put the National Guard on standby against any further violence on the city's mostly black north side. (Milwaukee County Sheriff via AP)
(The Associated Press)

A Milwaukee police officer who fatally shot a black man in August, sparking several nights of unrest on the city's north side, has been arrested on a sexual assault charge in an unrelated case, police said Thursday.

Officer Dominique Heaggan-Brown was arrested Wednesday, according to a statement released by the Milwaukee Police Department. The unidentified victim told police on Aug. 15 that Heaggan-Brown had sexually assaulted him while off duty, according to the statement.

The complaint was made two days after Heaggan-Brown fatally shot 23-year-old Sylville Smith, who police said was holding a gun when he was shot after a brief chase.

The department said an investigation found "additional allegations" that led to the charges against Heaggan-Brown. The criminal complaint wasn't publicly available Thursday morning, but the Milwaukee County Attorney's Office said it would be released later in the day.

Police said Heaggan-Brown is suspended and in custody, and they have launched an internal investigation. A police spokesman and the head of the Milwaukee police union didn't immediately return messages from the Associated Press seeking comment about the case.

Heaggan-Brown, 24, joined the police department in July 2010 as an aide. Like Smith, Heaggan-Brown is black. He was assigned to patrol the city's heavily minority north side.

Police Chief Edward Flynn has said that Smith was fleeing from a traffic stop when he was shot. Heaggan-Brown's body camera showed that Smith was shot after he turned toward an officer with a gun in his hand, according to investigators.

Smith's death sparked two nights of violence in the area's Sherman Park neighborhood, with several businesses burned. It also ramped up long-festering racial tension in Milwaukee.

The Wisconsin Department of Justice investigated Smith's death and has turned the case over to Milwaukee County District Attorney John Chisholm for a charging decision. It's not clear when a decision in that case will be made.